This invention relates generally to an apparatus and method for improving traction efficiency on rubber tired vehicles and reference is made to U.S. Pat. No. 4,224,972 the disclosure of which is incorporated herein by reference. The invention disclosed in this patent is expanded upon and its application on agricultural wheeled tractors is shown.
Agricultural equipment has been plagued with traction problems since the pneumatic tire replaced steel wheels in the 1930's. To alleviate this problem, adding ballast the drive wheels has been the accepted method for several decades, either in the form of auxiliary wheel weight, or liquid or dry ballast within the driving tires.
Since a typical tractor operates with many thousand pounds of ballast, improved traction is obtained at the expense of some 300 gallons of fuel annually to provide for hauling "ballast-power". In terms of energy, a staggering 40 trillion BTUs is consumed by each million farm tractors every year for the sole purpose of providing added traction ability. Furthermore, and equally significant, adding weight is only a half measure because in order for a pneumatic tire to be effective, it must operate at a considerable rate of slippage in order for the soil to build up thrust for propulsion, because a typical tire is sensitive to surface soil only.
This set of circumstances is in sharp contrast to characteristics exhibited by earlier tractors, having steel-lugged wheels. The steel-lugs not only penetrate into the stronger sub-surface soil layer without weight-adding, but are also capable of transmitting propulsion power at a much lower rate of slippage. As a result, if the same million of today's farm tractors were equipped with steel wheels, they would travel a distance of some 250 million miles further each year at the same number of wheel revolutions.
For example, in the 1910's Harry G. Ferguson observed that steel wheel tractors had such a pronounced over abundance of traction ability, that they were unsafe to the operation. If the implement met an obstacle, and the operator was not careful, the tractor would turnover backwards around its rear axle. To prevent this, Ferguson employed a brace between the tractor and implement, which is the upper link of the three-point hitch, now an industry standard.
In 1956, the Agricultural Tractor Test Code SAE 708c was approved and specifies in part, that for drawbar performance tests, maximum permissible slippage for tractors equipped with steel lugs is 7 percent. For tractors with pneumatic tires, 15 percent slippage is allowed, manifesting the difference in soil strength of sub-surface layer versus that of the surface soil.
In 1938, the SAE Cooperative Tractor Tire Test Committee concluded after extensive field tests that "The most important factor affecting drawbar performance is the soil itself." In 1978, Canadian researchers, in ASAE paper 77-1053, found that the conclusions of 40 years earlier by the above referenced Test Committee still stand.
Soil thrust is defined as the horizontal reaction produced by the ground when it is deformed by a vehicle wheel, and is generated by slippage of same. The associated shear action is resisted by soil strength, which provides the thrust necessary for propulsion. The maximum force required to shear the soil along the ground-contact area of a wheel, or the maximum thrust the soil can develop, is expressed with the equation: EQU T.sub.max =W.times.(C/P+tan .phi.)lb,
where two terms are vehicle-dependent:
W=weight carried by driving wheels; lb; PA1 P=W/A ground pressure, where A is ground contact area; sq. in. and, PA1 C=Coeff of soil cohesion; psi PA1 .phi.=angle of soil internal friction, deg.
two terms are soil-dependent:
From this it is clear that the amount of maximum soil thrust, equivalent to static drawbar pull, may be increased by adding weight (W) and ground contact area (A) in the form of ballast and oversize or dual tires, respectively. However, such measures will not improve the efficiency of transmitting power to the implement, as a wheel--comprising a pneumatic tire--interfaces with surface soil only.
In regard to developing thrust, soil-strata behave much like metals. As the rate of elongation in steel is only a third of that in aluminum to produce the same pull, the rate of deformation in sub-surface soil is less of that in surface soil to produce the same thrust. The difference in rates, is in accordance with their respective modulus. As experience has taught, soil becomes harder to deform as the depth from surface is increased. A lower rate of deformation corresponds to a lower rate of wheel slippage required for developing the same thrust.
Thus, to reduce the rate of wheel slippage, or reduce the loss in travel speed due to it, the characteristics of the stronger sub-surface soil must be utilized. With the need for movement on paved roads and utility, the pneumatic tire is still the preferred solution. The present invention overcomes its two serious shortcomings by employing a conveniently usable safe and lightweight accessory to obtain the advantages of a steel wheel without the disadvantages discussed above.